Preheat the oven to 180C/350F/Gas 4.
For the cauliflower, break the two cauliflowers into bite-sized florets and place into a baking tray. Drizzle with olive oil and season with salt and freshly ground black pepper, mixing until well combined. Roast for 20 minutes, or until golden-brown and tender.
For the vegetable curry, heat the ghee, or oil, in a large saucepan over a medium heat and fry the onion for 2-3 minutes, or until translucent.
Add the ginger, mustard seeds, curry leaves, turmeric, fenugreek, whole green chillies and chilli powder. Cook for 2-3 minutes, stirring frequently, until the mustard seeds pop and become aromatic. Add the potatoes and stir to coat in the spices.
Add the tomatoes, spinach leaves, sugar and 55ml/2fl oz water. Bring to a simmer and cook for 20 minutes, stirring occasionally.
Stir the roasted cauliflower florets into the curry and season to taste with salt and freshly ground black pepper. Add a squeeze of lemon juice to taste.
To serve, remove and discard the whole green chillies. Serve the curry alongside steamed basmati rice.